A basic concept of cellular radio telephone systems is that of enabling the allocated communicating channels to be reused several times in a given metropolitan or geographic area which results in increased spectral efficiency of the systems. However, because of the explosive growth in the number of users using the systems, the channel capacity and the dropped call rate in those cellular systems located in the large metropolitan areas have become important issues.
Various systems and technologies have been proposed for increasing the number of users in a system, and for reducing the dropped call rate. One such proposed technology comprises spread spectrum modulation with code division multiple access (CDMA), techniques. In such system, each potential user is given a unique code, and when the user wants to communicate, his or her code becomes part of the transmission.
The article "The Hybrid Transmission Scheme of a 900 MHz Digital Land Mobile Radio System", by Eizenhofer et al, IEEE, June 1991, discusses a hybrid transmission scheme for spread spectrum with cellular style radio systems in mind. Eizenhofer proposes a different modulation/coding scheme for the talk out (base to mobile) than he proposes for the talk back (mobile to base). Eizenhofer discusses methods for choosing codes that have good distance (low correlation) between them. Eizenhofer points out that codes with good distance between them need to be synchronized to prevent accidental correlations that would occur if they would not all start at the same time. Eizenhofer proposes using digitally coded voice signals. However, Eizenhofer does not mention the need for different reduced code sets at alternate base sites to control accidental interference between alternate sites.
The article "Performance Analysis of Spread Spectrum Packet Radio Network With Channel Load Sensing" by Abdelmonem et al, IEEE, January 1989, discusses the choice of codes in a large system, and concludes that because of practical limitations of designing a large set of orthogonal codes that a channel traffic loading sensor is important when traffic gets too high. They article does not mention the possibility of using smaller subsets of codes for different portions of the larger system.
In CDMA, users do not have to be assigned channels in advance, and many users can communicate simultaneously in a given band of spectrum. By using radio receivers that can search for selected codes, a signal within a broad band can be received even though that signal is weaker than other signals that are present. As an example, assume that in a given frequency band, a transmitter is coupled to a random noise source to transmit unwanted signals. A radio receiver assigned a given code can pick up a desired signal from a different transmitter transmitting the code and the desired signal, even though the desired signal is 20 DB weaker than unwanted signals.
In contrast to spread spectrum and CDMA technology, in conventional narrow band FM a desired signal needs to be about 15 DB stronger than the unwanted signals to provide good communication. Assume a cellular FM system of 25 KC width channels, so theoretically there could be 40 usable channels in one megacycle bandwidth. Commonly, cellular systems have a frequency plan reuse of 6 cells. Thus, in a one megacycle band, 40 (channels) divided by 6 (cells) results in about 7 users at a single site. U.S. Pat. No. 4,736,453 issued to the inventor herein, discloses a system and method for increasing the number of users to 9 to 12 users per single site. Thus, initial cellular systems can accommodate 7 users per site per megacycle, and cellular systems improved by U.S. Pat. No. 4,736,453 can increase the users to 9 to 12 per site per megacycle.
Theoretically, spread spectrum with CDMA technology can accommodate 30-40 users per site per megacycle. However, in actual practice the efficiency of spread spectrum with CDMA technology has been found to be significantly less than the indicated anticipated efficiency as will be described hereinbelow.